I haven't fruited Kwai Muk specifically. However, in my experience culture, nutrition, climate, soil, and ripeness of fruit all can influence quality and flavor. Sweetness of fruit can differ on the same tree depending on a combination of factors. I find that often later fruit on the same tree and season will be better. You can also have years where tree produces amazing fruit and same tree produce average fruit in other years. I don't judge quality of a plant on just one season's harvest.
Janet